Key West isn't a really exciting place. You can take a trolley tour - check out Hemingway's house and the Southernmost point of the USA, but if the weather isn't great (it was cold the day we were there), there aren't a ton of things to do. Scuba is OK and you can parasail, but again, it was too cold when we were there so we just walked around a bit, tried to have lunch at Blue Heaven (one of my favorite restaurants) then ended up at a creperie across the street.
Went with Peat Taylor to Dunn's River Falls then tubing on the White River. Lots of fun and Peat gets you there early so you beat the crowds. Everyone in Jamaica wants $$$ for everything, which is really annoying. A woman who was pouring free tastings of coffee at a shop near the port was really rude about wanting $ from me even though it was supposed to be a free sample.